In a packet switching network or Packet Switched Network, the data to be transmitted take the form of packets processed by installations of the network until reaching their destination. The set of packets to be transmitted constitutes a data stream.
An exemplary technology used in packet switching networks to convey data packets is the MPLS (MultiProtocol Label Switching) technology. The MPLS technology proposes that the header of the data packets be supplemented with one or more labels containing information allowing the installations of the network to determine the next hop that a packet must perform in order to reach its destination.
In such a network, the various installations of the network are connected together by means of MPLS paths called LSPs (Label Switched Paths).
An LSP path is set up from a head terminal installation (or Ingress installation), through intermediate installations and to a destination terminal installation (or Egress installation).
MPLS technology is described in greater detail in a document from the IETF (Internet standardization group, the abbreviation standing for Internet Engineering Task Force), referenced RFC 3031 (RFC standing for “Request For Comments”).
However, MPLS technology allows only packets complying with the IP (Internet Protocol) protocol to be processed.
In order to alleviate this drawback, the PWE3 (Pseudo Wire Emulation Edge to Edge) standardization group of the IETF defines a concept of virtual circuit making it possible to emulate a bidirectional point-to-point link between two installations of a packet switching network relying on IP/MPLS technology. Virtual circuits such as these, defined in the document RFC 3985, allow the transmission of data packets not complying with the IP protocol, such as for example data packets complying with the ATM protocol.
With reference to FIG. 1, a virtual circuit pw1 is set up between a first terminal installation PE1 disposed at the boundary of a packet switching network PSN and a second terminal installation PE2 also disposed at the boundary of the network PSN. A virtual circuit pw1 such as this is carried by an underlying LSP path, LSP1. An underlying path LSP1 such as this comprises a first link L1 set up between the terminal installation PE1 and an intermediate installation R of the PSN network. A second link L2 of the underlying path LSP1 is, for its part, set up between the intermediate installation R and the second terminal installation PE2. The first terminal installation PE1 constitutes at one and the same time a first end of the virtual circuit pw1 and of the underlying path LSP1. The second terminal installation PE2 constitutes at one and the same time a second end of the virtual circuit pw1 and of the underlying path LSP1. Once the virtual circuit pw1 has been set up, the terminal installation PE1 sends a data stream conveyed via the virtual circuit pw1 up to the terminal installation PE2.
In order to ensure continuity of service in the case of failure of the second link of the underlying path LSP1, it is known to implement the protection solution which is the subject of patent document WO 2008/037917, filed in the name of the applicant so as to ensure the continuity of the traffic between a source installation CE1 connected to the first terminal installation PE1 and a receiver installation CE2 connected to the second terminal installation PE2.
Such a solution consists in setting up a third link L5 between the intermediate installation R and a third terminal installation PE3 to which the receiver installation CE2 is also connected. The third link L5 is set up at the same time as the other constituent links L1 and L2 of the underlying path LSP1 on the initiative of the first terminal installation PE1.
Thus, when the intermediate installation R detects a failure of the second link L2 or of the second intermediate terminal PE2, it activates the third link L5 and shifts the data traffic onto the latter link.
Such a solution thus makes it possible to ensure continuity of service at the level of the underlying paths. However, such a solution does not make it possible to protect the virtual circuit pw1 in the case of failure of the second link of the underlying path. Indeed, a virtual circuit being a point-to-point link set up between a first terminal installation and a second terminal installation, a failure of the second link of the underlying path causes a break in the virtual circuit.
To alleviate this drawback, the PWE3 working group proposes a solution consisting in twinning the first virtual circuit pw1 with a second virtual circuit pw2 serving as backup virtual circuit so that, when the second link of the underlying path develops a fault the data packets are conveyed by means of the backup virtual circuit pw2, one end of which is constituted by a different output terminal installation.
Thus, with reference to FIG. 1, a second virtual circuit pw2 is set up between the terminal installation PE1, constituting a first end of the virtual circuit, and a second terminal installation PE3 constituting a second end of the virtual circuit. A virtual circuit pw2 such as this is carried by an underlying LSP path, LSP2. An underlying path LSP2 such as this comprises a first link L3 set up between the terminal installation PE1 and a second intermediate installation R′ of the network PSN. A second link L4 of the underlying path LSP2 is, for its part, set up between the intermediate installation R′ and the third terminal installation PE3.
In order to ensure continuity of service, the input terminal installation PE1 comprises means implementing a function for detecting a fault of the second link of the underlying path LSP1 or of the terminal installation PE2 and a function for shifting the data stream from the first virtual circuit pw1 to the second virtual circuit pw2.
When a failure occurs at the level of the second link of the underlying path LSP1, it is detected at the level of the input terminal installation PE1, for example on the basis of a message, sent across the network PSN, indicating the occurrence of a failure. Once informed of the failure, the input terminal installation PE1 triggers the shifting of the data stream from the first virtual circuit pw1 to the second virtual circuit pw2 thus ensuring the conveying of the data to the output terminal installation PE3.
However, a consequence of such a solution is a lengthening of the restore time in the case of failure of an output terminal installation or of a constituent link of an underlying path, this having a negative impact on service quality.